A turboprop engine comprises at least one propeller comprising a hub and blades that are supported by the hub and extend substantially radially outwards relative to the hub and to the axis of rotation of the propeller.
The turboprop engine is generally provided with a propeller pitch actuation system, also referred to as a propeller blade angular-pitch system. Adjusting the pitch of the propeller blades allows their efficiency to be improved while guaranteeing a propeller rotational speed for each flight phase.
Each blade can be rotated about an axis, which is generally radial, between a first emergency position, known as the feathering position, in which it extends substantially in parallel with the axis of rotation of the propeller, and a second position, in which it is sharply inclined relative to this axis. It can adopt any position between these two extreme positions.
In the prior art, the actuation system that is used is a hydraulic system, which is relatively complex and has several disadvantages. This system comprises an actuator of which one movable part is connected to the propeller blades so as to adjust their pitch.
The actuation system has to be able to provide not only the pitch control function, but also the emergency blade feathering function. The pitch actuation system thus comprises an auxiliary system for the emergency function.
A malfunction associated with a hydraulic leak, which is a mode common to the pitch control system and the auxiliary system, has to be covered. Without a pressure source, it is essential to add counterweights to the blades in order to provide the feathering function.
The pitch actuation system also has to provide protective functions in the event of overspeed, engine cut-out and failure of the FADEC (Full Authority Digital Engine Control) computer, and has to ensure that low pitches are limited during flight. A set of mechanical systems and hydraulic systems thus form part of the pitch actuation system for providing these functions in the prior art.
The pitch control system is also subject to extremely strict malfunction rate requirements, which entail redundancy and additional protection systems.
To conclude, the technology and the operating principle of a hydraulic propeller pitch actuation system are currently complex. These systems are integrated in a wide range of hydraulic components.
The present invention makes it possible to overcome these disadvantages and provides a solution to all or some of the aforementioned problems of the prior art.
The first problem (problem A) relates to the strict FHA (functional hazard assessment) requirements for pitch control, which entail robust architectures with redundancy.
The second problem (problem B) relates to the feathering function, which has to be able to be provided even after a failure of the pitch control means.
The third problem (problem C) relates to the risk of the movable part of the actuator locking. In a hydraulic system, a blade of the propeller is rotated by the translational movement of an eccentric at the root of the blade. Axial locking of the hydraulic cylinder is considered to be a failure.
Furthermore, in a hydraulic system, the rotation of the propeller is transmitted to the hydraulic actuator positioned in the rotating frame of reference (piston and body without angular movement). This cylinder is fed by pipes via a hydraulic spool positioned in the fixed frame of reference. In this hydraulic concept, the rotation of the propeller does not cause any offset of the propeller pitch. The fourth problem (problem D) relates to the management of this phenomenon.
Finally, the fifth problem (problem E) relates to the protective functions other than those covering the failure of the pitch control; these functions require additional mechanical and hydraulic devices in a hydraulic system of the prior art.